Begin with a class discussion about the
ways in which individual organisms—and
groups of organisms—interact with each other. Introduce or review
symbiosis as a relationship in which two organisms of different species
have a close association.

Organize students into pairs or groups and distribute the "Lean on Me" student handout. Assign one pair of organisms from the "Close Ties" list below to each group. Have students research information about the relationship between their assigned organisms.

Once students complete their research, have each group present its
findings. Ask the class to identify the similarities and differences among the organisms' associations. Create a chart with column headings for different types of relationship—such as mutualism, commensalism and parasitism—and have students place their pair of organisms in the appropriate column. Have students use their research to support their classification.

To conclude, ask students to consider the relationship between sulfur-oxidizing bacteria and tubeworms at hydrothermal vents. How would they classify this interaction? What other symbiotic relationships did they observe between organisms living near hydrothermal vents?

As an extension, have students determine where in the food web their pair of organisms fits. Discuss what the effects on the entire ecosystem might be if one or both of the organisms no longer existed.

Symbiosis is defined as a close
association between two organisms of different species. If one organism benefits and the other neither benefits nor is harmed, the interaction is called commensalism. If both organisms benefit, the interaction is called mutualism. If one organism is harmed and the other benefits, the interaction is called parasitism. Some relationships may be more than one kind.

Often the distinction between mutualistic and commensal relationships is not clear. Very close associations in which both organisms depend on each other for survival are mutualistic. Looser associations can be defined as either mutualistic or commensal. Students' research may differ from the chart below.

Some students may wonder how the predator and prey relationship is different from parasitism. Like predators, parasites take sustenance from another living organism. However, because a parasite's survival also depends on the survival of its host, it does not kill the host outright. A parasite lives on or in the host for some part of its life cycle, and the host may or may not die as a result of the association.

Organisms

Type of Symbiotic Relationship

Description of Relationship

shrimp and sea anemone

commensalism

The shrimp is immune to the stinging
tentacles of the sea anemone. By hiding in the sea anemone, the shrimp is protected from predators.

green alga and fungus (lichen)

mutualism

A green alga and fungus are dependent
on each other. The fungus gains nutrients
synthesized from the alga, and the alga receives water and nutrient salts from the fungus.

rhizobium bacteria and soybean plant

mutualism

The bacteria found on the roots of a soybean plant fix atmospheric nitrogen and make it available to the plant. The bacteria receives carbohydrates from the plant.

hermit crab and sea anemone

mutualism

The hermit crab is less likely to be eaten
by cuttlefish when an anemone rides on its shell. The anemone gains access to a wider feeding range.

oxpecker bird and hippopotamus

commensalism

The oxpecker bird eats ticks living on the hippopotamus's back.

tapeworm and dog

parasitism

The tapeworm attaches to the intestinal
wall of the dog and takes nutrients
consumed by the dog.

crocodile and Egyptian plover

mutualism

The Egyptian plover feeds on leeches and other scraps of food in the crocodile's mouth. The crocodile benefits because the plover cleans its teeth.

ant and acacia tree

mutualism

The ant burrows into a thorn of the acacia tree to live and eat sugar secreted by the tree. The ants benefit the tree by attacking predators.

cleaner fish and shark

mutualism

The cleaner fish feeds on parasites in the shark's mouth and gills.

tick and cow

parasitism

The tick burrows into the cow's skin to suck blood.

Book

Van Dover, Cindy Lee. The Octopus' Garden. Reading, Mass.: Addison-Wesley, 1996.
The author, a former submersible pilot, describes the difficult conditions under which scientists work as they explore the bottom of the sea.

Article

Tunnicliffe, Verena. "Hydrothermal-Vent Communities of the Deep Sea." American Scientist (July/August 1992): 336-349.
Describes communities found near hydrothermal vents including examples of animals that use chemosynthesis as their energy source.

Web Sites

NOVA Online—Into the Abysshttp://www.pbs.org/nova/abyss/
Provides background information on the research expedition featured, life in deep ocean environments, technology used to raise a deep-sea vent, location of global vent sites and a timeline of undersea exploration.

Revel Projecthttp://www.ocean.washington.edu/outreach/revel/
This initiative, Research and Education: Volcanoes, Exploration and Life (REVEL), promotes interaction between teachers and scientists. Selected teachers participate in seagoing research expeditions.

The "Lean on Me" activity aligns with the following National Science Education Standards:

Grades 5-8

Science Standard C:Life Science

Populations and ecosystems

Populations of organisms can be categorized by the function they serve in an ecosystem. Plants and some microorganisms are producers—they make their own food. All animals, including humans, are consumers, which obtain food by eating other organisms. Decomposers, primarily bacteria and fungi, are consumers
that use waste materials and dead organisms for food. Food webs identify the relationships among producers, consumers and decomposers in an ecosystem.

Grades 9-12

Science Standard C:Life Science

The interdependence of organisms

Organisms both cooperate and
compete in ecosystems. The interrelationships and interdependencies of these organisms may generate ecosystems that are stable for hundreds or thousands of years.

Volcanoes of the Deep

Original broadcast:March 30, 1999

Deep-Sea Vents and Life's Origins
Travel in this Teachers' Domain video segment (3m 30s) to the rich ecosystems at the seafloor near mid-ocean ridges.